You just answered your own question.
"Why does romeo get banished after killing Tyblat?"
she was suprised how romeo killed her cousin
Yes.
Romeo killed him
He's banished from Verona.
Getting banished from verona
Romeo is actually banished to Mantua for killing Tybalt, he didn't want to travel there.
The prince- he was banished from Verona for killing Tybalt.
Romeo is supposed to live out his banishment in the nearby town of Mantua. This is where he flees to after being banished from Verona for killing Tybalt.
Well, after Romeo get banished for killing Tyblat (who killed Romeo's cousin, Mercutio) by the Prince, who was upset because Mercutio was he cousin, Romeo goes to Friar Lawrence's cell and weeps. Juliet's nurse shows up and tells Romeo that Juliet wants to see him for a last farewell. He shows up and I can guess that they have sex. Juliet's mother knocks on her door and Romeo leaves. So the Answer to your question would be Romeo.
Romeo because he is banished from Verona after killing Tybalt.
In Act III Scene I, Romeo is banished by the prince after killing Tybalt.
Romeo will stay in Mantua while he is banished from Verona. He flees Verona after being exiled by Prince Escalus for killing Tybalt. The Friar arranges for Romeo to stay in Mantua until things calm down.